Permanent magnet system for focusing an electron beam in a travelling wave tube



May 4, 1965 P. MEYERER 3,182,234

PERMANENT MAGNET SYSTEM FOR FOGUSING AN 7 ELECTRON BEAM IN A TRAVELLING WAVE TUBE Filed Oct. 50, 1961 4 Sheets-Sheet l s 'i la ZN/ v 3 N V I is 1 7 i W T W h I, "I N y 4, 1965 P. MEYERER 3,182,234

PERMAN MAGNET SYSTEM FOR F0 ING AN E TUBE 4 Sheets-Sheet 2 ELECTR BEAM IN A TRAVELLING Filed Oct. 30. 1961 Fig.5

Fig.7 ,E-I-F y 4. 1 65 P. MEYERE 3,182,234

PERMANENT M T TEM F F CU G AN ELECTRON B N RAV WA TUBE Filed Oct. 30, 1961 4 Sheets-Sheet 5 May 4, 1965 P. MEYERER 3,182,234

PERMANENT MAGNET SYSTEM FOR FOCUSING' AN ELECTRON BEAM IN A TRAVELLING WAVE TUBE Filed 001' 30, 1961 4 Sheets-Sheet 4 Fig.1]

G-Z-H United States Patent F 3,182,234 PERMANENT MAGNET SYSTEM FOR FOCUSING ANBEJLECTRON BEAM IN A TRAVELLING WAVE U 4 Paul Meyerer, Munich, Germany, assignor to Siemens &

Halske Aktiengesellschaft, Berlin and Munich, Germany, a corporation of Germany Filed Oct. 30, 1961, Ser. No. 148,655 Claims priority, application Germany, Feb. 22, 1961, S 72,657 4 Claims. (Cl. 317-200) This invention is concerned with a permanent magnet arrangement or system for focusing an electron beam along an extended path, especially in connection with travelling wave tubes.

In permanent magnet arrangements for producing magnetic fields for focusing electron beams of travelling wave tubes, the upper limit of the magnetic field strength is determined by the permissible weight of the magnet or by the saturation of the soft iron pole pieces. The underlying cause exists in the magnetic scattering flux.

The problem underlying the invention is to produce a permanent magnet arrangement for focusing electron beams over an extended path, with reduction of the magnetic scattering field.

It is for the solution of this problem proposed to provide in accordance with the invention, in connection with a permanent magnet arrangement for producing a magnetic field for the focusing of an electron beam over an extended path, especially for travelling wave tubes, hard magnetic ferrite of great coercive power within the region or area of the greatest magnetic scattering flux.

By use of the invention, a higher field strength can be obtained with a given magnet weight or a reduced magnet weight can be obtained with a given desired field strength.

Details of the invention will now be described with reference to the accompanying drawings.

FIGS. 1 and 2 show in schematic representation a magnet arrangement for producing a transverse magnetic field, for example, for an M-type travelling wave tube, FIG. 2 thereby showing a section taken along the line A-B of FIG. 1;

FIG. 3 designates the pertinent dimensions involved in the arrangement illustrated in FIGS. 1 and 2;

FIGS. 4 and 5 show an embodiment of a permanent magnet arrangement according to the invention, for producing a spatial periodic magnetic field, FIG. 5 thereby representing a section taken along the line (3-1) of FIG. 4;

FIGS. 6 and 7 indicate an arrangement similar to the one shown in FIGS. 4 and 5, FIG. 7 thereby representing a section taken along line EF of FIG. 6;

FIGS. 8 and 9 explain the operation of an embodiment of a shielded permanent magnet arrangement;

FIGS. 10 and 11 indicate in diagrammatic manner a permanet magnet arrangement, for example, for a carcinotron for millimeter waves, FIG. 11 thereby representing a section taken along line GH of FIG. 10; and

FIGS. 12 and 13 show an embodiment for producing a homogenous magnetic field.

As noted above, FIGS. 1 and 2 show in schematic representation a magnet arrangement for producing a transverse magnetic field, for example, for an M-type travelling wave 3,182,234 Patented May 4, 1965 tube. The two permanent magnets 1 and 2, consisting, for example, of Alnico V, are upon one side magnetically shunted by a soft iron member 3. The free ends of the baror rod-shaped permanent magnets 1 and 2 are extended by means of pole pieces 4 and 5 between which is to be produced the magnetic working field with the lines of force 6. The magnetic scattering flux (field lines 7) flows outside as well as within the space between the two permanent magnets 1 and 2, the greatest scattering flux density occurring in the immediate vicinity of the useful working flux which is present between the pole pieces 4 and 5.

Upon providing, as indicated in FIG. 3, a hard magnetic ferrite 8 of great coercive power in the region of the greatest magnetic scattering flux, such scattering flux will be suppressed between the two permanent magnets 1 and 2. There is thereby produced an auxiliary permanent magnet which has approximately the same magnetic power as the two permanent magnets 1 and 2.

The magnetic gain obtained in an arrangement according to FIG. 3, as compared with an arrangement according to FIGS. 1 and 2, shall now be determined in approximation. The magnetic field strength is with a given magnetic induction B in known manner obtained from the permeability ,u of the characteristic working point of the magnet upon the hysteresis curve. This permeability ,u can also be calculated from the geometric dimensions of the magnet arrangement according to the known formula The first member of the formula thereby denotes the permeability of the permanent magnets, the second member the permeability of the air gap and the third member the permeability of the scattering flux F is the cross sectional area of the permanent magnets and F is the area of the air gap. The significance of the remaining parameters can be derived from FIGS. 2 and 3.

The third member of the above noted formula, that is, the permeability of the scattering flux, can be made approximately zero by the provision of the hard magnetic ferrite 8, whereby the total permeability u is reduced. As a result of this reduction of permeability the field strength in the air gap can at a given magnetic power, be increased, or the magnetic weight can be reduced, retaining the same field strength.

A numerical example, noting 1 :4 cm., l =2 cm., [2:10 cm.,

a=3 cm., F =30 cm. F :1O cm. will result in (MA)1 od) (y, airgap) (,u scattering flux) In order to obtain without the hard magnetic ferrite a permeability of 18.4, the cross sectional area of the '3 permanent magnets would have to be increased from 10 to 15 cm. Accordingly, in the case of the above noted numerical example, the gain with respect to magnetic weight amounts to the factor 1.5.

The features of the invention can be particularly advantageously applied in the case of known magnet systems for producing a magnetic field which alternates in electron radiation direction, that is, a spatially periodic magnetic field.

FIGS. 4 and 5 show an example of an embodiment of 'a permanent magnet arrangement according to the invention, for producing a spatially periodic magnet field. There are provided permanent magnets 9, lit] and 11, 12 which are arranged in pairs and magnetized in a plane perpendicular to the electron radiation axis, such magnets extending along the electron beam parallel therewith. The magnets 9 and 15 which are mutually superposed in vertical direction are thereby, as seen in electron radiation direction, successively interconnected by means of pole pieces 13, 14, and 15. The magnets 11 and 12 which are arranged horizontally with respect to the magnets 9 and it), have with respect to the latter opposed polarity and are likewise mutually interconnected by means of pole pieces 16, 17. The pole pieces 16 and 17 shall have, just as the pole pieces 13, 14, in the vicinity of the permanent magnets a larger cross-sectional size than in the vicinity of the electron beam. Pole pieces which interconnect the magnets 9, alternate, as seen in electron radiation direction, with pole pieces which interconnect the magnets 11, 12, thereby producing alternating magnetic field. Between the individual pole pieces are arranged hard magnetic ferrite rings 18, 19, 20, the effect of which is that the pole pieces are not so strongly saturated. The operatively effective power to be applied to the pole pieces and therewith the field strength oriented in the electron radiation direction, can thereby be considerably increased.

The permanent magnet arrangement shown in FIGS. 6 and 7, is similar to the one illustrated in FIGS. 4 and 5. The permanent magnets 20, 21, 22 and 23 which are made, for example, of Alnico, are arranged symmetrically to the electron radiation axis so as to form the sides of a square, whereby identical poles meet at the corners of the square, and opposite corners of the square therefore have the same polarity. The corners with identical polarity are in horizontal direction interconnected over the pole pieces 24, 7.5 and 26. The pole pieces 27, 23 and 29 establish connection with identical poles in vertical direction. Between the individual pole pieces 28, 25 and 25, 28', respectively, which form the horizontal connection, are again arranged hard magnetic ferrite rings 30, 30' and 31.

The invention is also advantageously applicable in the case of shielded permanent magnet arrangements. The operation of an embodiment of a shielded permanent magnet arrangement shall now be explained with reference to FIGS. 8 and 9.

The arrangement shown in FIG. 8 is similar to the one illustrated in FIGS. 6 and 7 except for the shielding casing 32, made of soft iron sheet material, which encloses the various parts. As indicated by the lines of force 33, there will result a scattering flux directed particularly from the pole pieces 24, 27, 26 and 29 to the shielding casing 32. This scattering flux weakens the power of the permanent magnets 20, 21, 22 and 23. Numerals 34, 35, 36, 37, in FIG. 9, indicate hard magnetic ferrite provided within the areas of the greatest scattering flux between the magnet system and the shielding envelope 32., namely, between the envelope corners and the corners formed by the magnets, such ferrite being effective to practically completely suppress the scattering flux. Accordingly, the permanent magnets are, despite the shielding envelope or housing, practically unloaded.

The invention is by no means limited to systems for producing alternating magnetic fields; it may also be employed, especially in connection with magnetic systems for producing a homogenous magnetic field oriented in the electron radiation direction. FIGS. 10 and 11 show such an arrangement in schematic representation, for example, for a carcinotron for millimeter waves, FIG. 11 showing a sectional view taken along lines G-H in FIG. 10. At the beginning and at the end of the magnet system, which is to extend over the discharge path of the tube, there are respectively arranged in a plane perpendicularly to the electron radiation axis, two pairs of permanent magnets 34, 35 and 36, 37. The poles of these magnets which are adjacent to the electron beam, are respectively identical, but are in case of the magnets 34, 35 arranged at the beginning of the discharge path, of a polarity opposite to that of the permanent magnets 36, 37 arranged at the end of the discharge path. At the side facing away from the electron beam, the permanent magnets 34, 36 and 35, 37, respectively, are shortcircuited over soft iron members respectively indicated at 33 and 39. The poles neighboring on the electron beam are respectively interconnected over pole pieces 4t), 41 which are provided with an opening for the passage of the discharge vessel. The operatively effective flux extends only over the area between the two bores in the pole pieces 40 and 41. The remaining space between the pole pieces 40 and 41 is permeated by a considerable scattering flux. The scattering flux is to a far reaching extent suppressed by two hard magnetic ferrites 42 and 43, arranged symmetrically to the electron radiation axis on both sides thereof.

The invention is applicable in connection with a shielded permanent magnet. arrangement for the production of spatially alternating fields as it is applicable in connection 1 with a shielded permanent magnet arrangement for'the production of a homogenous magnetic field. FIGS. 12 and 13 show an example, FIG. 13 illustrating a crosssectional view of the structure along line 1-K of FIG. 12.

For the sake of simplicity, only one half of the arrangement which is symmetrical to the axis 4-3, is shown in FIGS. 12 and 13. The permanent magnets 44 are to serve for the production of a homogeneous magnetic field. These permanent magnets are symmetrically surrounded by a housing 45 made of soft iron sheet, for the magnetic shielding thereof. Differently shaped magnet means, for example, a barrel shaped permanent magnet may take the place of the magnets 44 A strong scattering flux will again result in such arrangement, especially between the magnets and the shielding housing 45, in the regions of the poles of the permanent magnets 44. This scattering flux, indicated by the lines of force 46, which reduces the operably effective magnetic field strength, can be suppressed to a far reaching extent by means of hard magnetic ferrite 47 disposed within the region of the strongest scattering flux. The hard magnetic ferrite 47 is wedgeshaped in approximation to the configuration of the permanent magnet 44, the diameter of which increases centrally thereof so as to compensate or to equalize in known manner the magnetic action over the length of the magnetic field.

Changes may be made within the scope and spirit of the appended claims which define what is believed to be new and desired to have protected by Letters Patent.

I claim:

1. In a permanent magnet arrangement for producing a magnetic field for focusing an electron beam over an extended path, having permanent magnets which are surrounded by a shielding housing made of soft iron; the improvement which comprises, arranging in the regions of great magnetic scattering flux, between the permanent magnets and the shielding housing, ferrites of great coercive power which are magnetized with a polarization oriented opposite to the direction of the scattering fiux.

2. A permanent magnet arrangement according to claim 1, wherein said hard magnetic ferrites are arranged at both ends of at least one permanent magnet, which extends parallel to the electron radiation axis, between the permanent magnet and the shielding housing surrounding the permanent magnet.

3. A permanent magnet arrangement according to claim 2, for producing a magnetic field which alternates in the direction of said extended path, wherein the permanent magnets are disposed along the sides of a closed square, identical poles of the permanent magnets abutting at the corners of the square, the hard magnetic ferrites being arranged between the corners of the square formed by the permanent magnets and the corners of the shielding housing, the latter likewise being square-shaped in transverse cross-section.

4. A permanent magnet arrangement according to claim 2, for producing a homogeneous magnetic field, comprising four permanent magnets which extend sym- References Cited by the Examiner UNITED STATES PATENTS 2,936,408 5/60 Bennetot 3'l7200 X 2,983,840 5/61 Iperen 317-200X IOHN F. BURNS, Primary Examiner. GEORGE WESTBY, E. JAMES SAX, Examiners. 

1. IN A PERMANENT MAGNET ARRANGEMENT FOR PRODUCING A MAGNETIC FIELD FOR FOCUSING AN ELECTRON BEAM OVER AN EXTENDED PATH, HAVING PERMANENT MAGNETS WHICH ARE SURROUNDED BY A SHIELDING HOUSING MADE OF SOFT IRON; THE IMPROVEMENT WHICH COMPRISES, ARRANGING IN THE REGIONS OF GREAT MAGNETIC SCATTERING FLUX BETWEEN THE PERMANENT MAGNETS AND THE SHIELDING HOUSING, FERRITES OF GREAT COERCIEVE POWER WHICH ARE MAGNETIZED WITH A POLARIZATION ORIENTED OPPOSITE TO THE DIRECTION OF THE SCATTERING FLUX. 